"Mainstream Linux distributions typically default to one of two desktop environments, KDE or GNOME. Both of these environments provide users with an intuitive and attractive desktop, as well as offering a large raft of multimedia software, games, administration programs, network tools, educational applications, utilities, artwork, web development tools and more. However, these two desktops focus more on providing users with a modern computing environment with all the bells and whistles featured in Windows Vista, rather than minimising the amount of system resources they need. For users and developers who want to run an attractive Linux desktop on older hardware, netbooks, or mobile internet devices, neither KDE or GNOME may be a viable option, as they run too slowly on low spec machines (such as less than 256MB RAM and a 1 GHz processor). This article seeks to identify the best lean desktops for Linux, for users that have old or even ancient hardware."

Yes, Openbox is a great little fill in as Window manager. I've got it in KDE3 and it's making it much snappier. It's also giving you the option to start KDE normally with KWin set up for transparency and other effects and if you don't want them 'cos you need to get things done a little faster just start KDE-Openbox session.
It's also great on its own with lxpanel/fbpanel and idesk for desktop icons. Gkrellm will take care of all additional monitoring needs that the 'Big Three' are giving you applets for.